1. Field of the Invention
This invention relates generally to a new method and apparatus for computed tomography scanning. More particularly, it relates to a new x-ray beam filtering technique useful for reducing adverse affects caused by chromatic artifacts.
2. Prior Art
In recent years the art of computed tomography scanning has enabled medical personnel to gain information about the internal structure of a patient that is not available with other procedures. A conventional radiograph is a two dimensional shadow image of a three dimensional subject. The depth dimension is not apparent because all interior portions of the patient are imaged in a single plane. As a consequence, a conventional radiograph often fails to show, in the direction of x-ray propagation, the spacial location of a condition existant within a patient.
In computed tomography, an image of a cross-sectional plane of a subject or patient is developed by sequentially directing radiation through the patient from a plurality of origins. Detectors are used to detect x-radiation intensity after it has passed through the patient. From these intensity values it is possible to reconstruct an image of the cross section of interest. Various reconstruction algorithms have been developed, each requiring intensity readings from a number of different orientations. These readings are modified according to those algorithms and transmitted to imaging electronics which provide a density mapping of the patient.
When the computed tomography reconstruction algorithm has been performed the diagnostician has available a representation of a patient cross section or slice which indicates variations in density within the patient. If a number of these slices of cross sections of the patient are obtained, the position and location of various organs and tissue can be analyzed with great precision.
CT Scanning apparatus has evolved from a so-called first generation CT Scanner to improved fourth generation scanners. The first generation scanner included one "pencil" beam source of x-rays and a single x-ray detector. The source-detector pair translated and rotated about the patient so this first generation scanner was called a translate-rotate (TR) machine. Second generation machines are T.R. machines with plural beams and detectors while third generation machines orbit a source and a set of detectors. Fourth generation machines include a stationary, annularly disposed, detector array comprising hundreds of individual detectors. One source of radiation rotates about the patient thereby irradiating the cross section of interest from a variety of positions. The stationary detectors transmit intensity data to the imaging electronics for reconstruction processing.
As the art of computed tomography scanning has matured, certain techniques for improving the CT image has been formulated. One improvement has been the reduction of various artifacts which degrade image quality. These artifacts typically appear as lines or streaks which suggests structure but which in fact correspond to no density variation within the patient. One such artifact or aberation is a so-called "chromatic artifact". Chromatic artifacts occur in the image due to the fact that the x-radiation emitted from the x-ray source comprises radiation of more than one energy. When x-radiation is emitted from an x-ray tube the range or distribution of energies comprises a skewed gaussian energy distribution. The skewing of the gaussian distribution is concentrated toward a low energy side so that many photons comprising the x-ray beam are concentrated about a mean energy and an exponentially decreasing number of photons have higher energies.
Examples of chromatic artifacts or aberations within computed tomography scanning are known. If a CT scan is made of a completely uniform body such as a cross section of water, the computed tomography reconstruction should show a uniform cross section representing the constant density of the water. In fact, when a computed tomography scan is taken of a uniform body such as water, the edge portions of this section appear lighter than the interior.
One useful CT application is a so-called brain scan. It is known from extrinsic examination that brain tissue is fairly uniform in density. It is also known that a CT scan of the brain tissue results in lighter image quality around the edge portions of the brain with darker interior portions. From the experience in scanning a water phantom, it is theorized, therefore, that the difference in density seen in a typical brain scan is a result of the non-monochromatic nature of the scanning radiation and not due to variations in brain tissue density.
The chromatic artifact problem also appears when bone structures are examined. In a scan of the human skull dark streaks appear between projections of bone. As is the case in brain tissue scanning, it is known that these dark streaks may not correspond to structural or density variations. If the doctor or medical researcher knows these streaks or variations are artifacts, he can take this factor into account when diagnosing. If, however, the doctor is examining a region about which he has no prior knowledge, these streaks and nonuniformities can confuse and hide the true structure and condition of the patient.
In addition to concern over artifact problems, research in CT scanning has been aimed at increasing the amount of useful information CT scanning procedure can provide. In addition to patient density data, it is now possible for the CT scan to provide a mapping of atomic number and electron density variations in the patient. As is known, the atomic number is variable depending upon the material comprising the particular cross section being scanned. The electron density is to some extent also dependent upon this material but also varies with the packing density and configuration of the material.
Electron density variation information is a valuable aid in radiation therapy. Recently developed calculations for the proper dosage required in cancer treatment require that the electron density within the area under radiation therapy be known to an accuracy of approximately 2 percent. Advances in the CT scanning technique have provided this degree of accuracy in electron density mapping and aided the radiotherapist to prescribe proper dosage treatments.
Knowledge regarding the atomic number variation within the patient can have significance in two important diagnostic areas. Knowledge concerning the atomic number variations can lead to differential diagnoses of tumors and cysts within the patient. Perhaps even more importantly, atomic number variations can prove important in injection contrast diagnostic procedures.
When an atomic number density variation image is created, the effects of injected agents such as iodine and xenon are magnified. This follows since the injected elements typically have atomic numbers much greater than the atomic numbers represented in human tissue. Since contrast injection procedures are frequently utilized in CT scanning, any enhancement of the visualization of such agents significantly aids diagnostic procedures.
Heretofore, proposals for chromatic artifact reduction and variation mappings of electron density and atomic number were achieved at the expense of slower scanning times and increased patient exposure to x-radiation. To provide the imaging electronics with enough information to reduce the artifacts and to diversify information available from a CT scan, prior proposals necessitated two CT scans for a given scanner position relative to the patient. One scan was conducted at a first average x-ray energy and then a second scan was conducted at a second energy different from the first scan energy. The intensity data from these two scans was then modified according to known data processing techniques to reduce artifacts and to yield the atomic number and electron density variation information.